Charge pumps are devices that operate as power supplies for electronic circuitry. They provide a controlled output voltage that is higher than the input voltage of the charge pump. Charge pumps are generally implemented by cascading stages that include energy injection capacitors and charge transfer elements. Four-phased multi-stage charge pumps are generally considered to be one of the most efficient pump architectures known in the art and are, accordingly, widely utilized in the art.
Energy is injected into a given stage by a driver which provides a clock signal to the input of the capacitor(s). This signal oscillates between a positive supply rail (for example, VDD) and a negative supply rail (for example, GND). The voltage at the output of the capacitor is boosted by the voltage swing between the supply rails (for example, from GND to VDD). The charge transfer element (often a transistor or a diode) transfers the accumulated charge to the next stage. Each charge pump stage boosts the voltage in proportion to the voltage swing of the driver such that the output of the pump is a multiple of the voltage swing.
Known prior art methods of regulating the output voltage of charge pumps have varied limitations that significantly affect the overall efficiency of the charge pump's operation.
U.S. Pat. No. 5,276,646 of Kim et al., which is incorporated herein by reference in its entirety, describes a system and method for providing a constant voltage at the output of a charge pump. The system and method taught in Kim continuously measures the output of the charge pump and enables or disables the operation of the charge pump based on the value of the measured output voltage being above or below the desired output voltage. This on/off method causes a large delay between the actual reading of the output voltage and the corrective action taken with respect to the operation of the charge pump such that a ripple effect of as high as 1 volt is introduced at the output voltage of the charge pump. Moreover, when the charge pump is turned on, large substrate noises occur.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.